A known position sensing device is configured to detect the position of a movable member according to pulse signals in multiple phases sent from an encoder accompanied with rotation of the movable member or accompanied with linear movement of the movable member. For example, JP-A-1-223311 discloses a position sensing device including an encoder equipped with a pulse output unit configured to send three-phase pulse signals with rotation or linear movement of a movable member. The three-phase pulse signals have a phase difference of 120 degrees thereamong. With this configuration of JP-A-1-223311, the encoder causes the pulse output unit to send the three-phase pulse signals with specific combination patterns. Herein, when a malfunction occurs in the pulse output unit, the three-phase pulse signals sent from the pulse output unit show a pattern (in-malfunction pattern) in which all the three-phase signals coincide with each other at the high level or at the low level.
The encoder of JP-A-1-223311 is configured to detect the pattern (in-malfunction pattern), in which all the three-phase pulse signals coincide with each other at the high level or at the low level, thereby to detect a malfunction in the pulse output unit. Nevertheless, it is noted that, the position sensing device of JP-A-1-223311 cannot continue detection of the movable member appropriately after a malfunction occurs in the pulse output unit of the encoder.
Herein, JP-A-1-223311 further discloses a servomotor equipped with the above-described encoder and a servomechanism equipped with the servomotor. In this servomechanism, the encoder is configured to send three-phase pulse signals with rotation of a sending screw rotated by the servomotor. The position sensing device is configured to detect the rotary position of the sending screw (machinery sending stand) according to the three-phase pulse signals. Rotation of the servomotor is feedback-controlled according to the rotary position detected with the position sensing device. Herein, in a case where a malfunction occurs in the pulse output unit of the encoder, the position sensing device cannot detect the rotary position correctly, and consequently, rotation of the servomotor cannot be continued. When rotation of the servomotor cannot be continued, it is impossible to drive the machinery sending stand of servomechanism.